Hologram recording/reproducing system

ABSTRACT

A hologram recording and reproducing system having compatibility has: a supporting unit for freely attachably supporting a recording medium including a photosensitive material; a signal light-generating unit for projecting a coherent light beam modulated according to a predetermined data into a recording medium and generating a diffraction grating by providing a three-dimensional light interference pattern in the recording medium; a detector unit for detecting and photoelectrically converting a diffracted light from the diffraction grating; and a demodulating unit for demodulating a predetermined data from an output from the detector unit. The detector unit has an intermediate data-generating unit for generating an intermediate data, and the demodulating unit has a conversion table in which the intermediate data and the predetermined data are uniquely associated, and demodulates the predetermined data by performing a computation based on a correlation in the conversion table.

TECHNICAL FIELD

The present invention relates to a holographic recording medium and arecording and reproducing system utilizing this.

BACKGROUND ART

To date, hologram recording systems have been known as a digitalinformation recording system that utilizes the principle of hologram. Afeature of this system is to record an information signal into arecording medium as a change in a refractive index. Photorefractivematerials such as a single crystal lithium niobate or the like are usedfor the recording medium. In a hologram recording medium, data can berecorded and reproduced in the units of two-dimensional plane pages, andmultiplexed recording is possible by using a plurality of pages. Theoutline of the recording medium system is explained below.

At the time of recording, in a conventional 4f system hologram recordingand reproducing apparatus, a laser light beam 12 emanating from a laserlight source 11 is split into lights 12 a and 12 b by a beam splitter13, as shown in FIG. 1. The light 12 a is shaped into a substantiallycollimated light, the beam diameter of which is enlarged by a beamexpander BX, and is projected onto a spatial light modulator (SLM) suchas a transmissive-type TFT liquid crystal display (Thin Film TransistorLiquid Crystal Display) (hereinafter also referred to as “LCD”) panel orthe like. An encoder 25 converts a digital data to be recorded in arecording medium 10 into an bright and dark dot-pattern image on a planeand rearranges it into a data array of, for example, 480 verticalbits×640 horizontal bits. The encoder generates a unit-page series dataand sends out the data to the spatial light modulator SLM.

When the light 12 a transmits through the spatial light modulator SLM,it is light-modulated and turned into a signal light containing a datasignal component. The signal light 12 a containing the dot patternsignal component passes through a Fourier transform lens 16, which isspaced apart by its focal distance f, and the dot pattern signalcomponent is Fourier transformed. Then, the light is gathered into arecording medium 10.

On the other hand, the light beam 12 b split by the beam splitter 13 isguided as a reference light into the recording medium 10 by mirrors 18and 19, and it intersects the light path of the signal light 12 a withinthe recording medium 10, forming a light interference pattern. Thus theentirety of the light interference pattern is recorded as a change inthe refractive index (refractive index grating). In addition, it becomespossible to record a plurality of two-dimensional plane data with anglemultiplexing by varying the incident angle of the reference light 12 bonto the recording medium 10.

At the time of reproducing, inverse Fourier transform is performed toreproduce the dot pattern image. As shown in FIG. 1, for example, thelight path of the signal light 12 a is blocked by the spatial lightmodulator SLM so that only the reference light 12 b is projected ontothe recording medium 10. The reference light 12 b is controlled by themirror driven in the position and angle thereof with a combination ofthe rotation and linear movement so that the incident angle thereofresults in the same as that of the reference light at the time when thepage to be reproduced has been recorded. A reproduced light thatreconstructs the recorded light interference pattern appears on a sideof the recording medium 10 that is opposite the side thereof that isirradiated with the reference light 12 b. When this reproduced light isguided to the inverse Fourier transform lens 16 a and is inverseFourier-transformed, the dot pattern image can be reconstructed.Further, this dot pattern image is received by a photo-detector 20 suchas a charge coupled device (CCD) or the like at the focal distanceposition, and the image is reconverted into an electrical digital datasignal. Thereafter, the data signal is sent to a decoder 26, and theoriginal page data is reproduced.

In the recording and reproducing system shown in FIG. 1, according tothe rules of Fourier transform and inverse Fourier transform, thetransmitted light for, for example, the portion of the image data “A” asshown in FIG. 2( a) that is displayed on the spatial light modulator SLMis Fourier-transformed and recorded into the recording medium as aninterference pattern of Fourier transform pattern, and the image of theimage data A that has been inverse Fourier-transformed as shown in FIG.2( b) is reproduced on the CCD 20 from the recording medium illuminatedwith the reference light. Therefore, the conventional recording andreproducing system necessitates a CCD 20 that is similar to the spatiallight modulator SLM with 480 vertical bits×640 horizontal bits and hasthe same resolution. The precondition is that the recording andreproducing system uses a fixed conversion rule for the recording systemand the reproducing system to perform recording and reproducing.

For this reason, it is required for the conventional recording andreproducing system to keep optical distortion, deviation of the signalimage, or the like that occurs in the Fourier transform optical system,the inverse Fourier transform optical system, and other optical systems,within a predetermined specified value range. This requires suchcomponents as high-precision lenses or the like for the optical systems,and moreover a high-precision relative position adjustment is necessary.Furthermore, since the transfer of pixel data is performed, an expensivedetector such as a CCD or the like is required in order to performhigh-speed data transfer.

Accordingly, an example of the problem that the present inventionintends to solve is to provide a hologram recording and reproducingsystem that does not require an inverse Fourier lens.

DISCLOSURE OF THE INVENTION

A hologram recording and reproducing system of the invention has asupporting unit for freely attachably supporting a recording medium(including a photosensitive material such as a photorefractive polymer,a hole burning material, a photochromic material, etc.); a signallight-generating unit for projecting a coherent light beam modulatedaccording to a predetermined data into the recording medium andgenerating a refractive index grating by providing a three-dimensionallight interference pattern in the recording medium; a detector unit fordetecting and photoelectrically converting a diffracted light from therefractive index grating; and a demodulating unit for demodulating apredetermined data from an output from the detector unit, the hologramrecording and reproducing system characterized in that: the detectorunit has an intermediate data-generating unit for generating anintermediate data, and the demodulating unit has a conversion table inwhich the intermediate data and the predetermined data are uniquelyassociated, and demodulates the predetermined data by performing anoperation based on a correlation in the conversion table.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view showing the configuration of aconventional recording medium system.

FIG. 2 is a view for illustrating image data that appears on a spatiallight modulator and a CCD.

FIG. 3 is a diagrammatic view showing the configuration of an embodimentof a recording medium system according to the invention.

FIG. 4 is a view for illustrating a Fourier transform pattern thatappears on a light-receiving face of a photo-detector in the vicinity ofthe Fourier plane.

FIG. 5 is a diagrammatic view showing the configuration of anotherembodiment of the recording medium system according to the invention.

FIG. 6 is a view for illustrating a spot of a reference light beam thatappears on a position sensor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, embodiments of the present invention are explained withreference to the drawings.

In a hologram recording and reproducing system of the presentembodiment, an intermediate data is reproduced in advance and thereproduced intermediate data is computed based on a correlation in apredetermined conversion table that has been stored in advance, todemodulate an original data, in the case in which conversion rules aredifferent between the recording system and the reproducing system. Thecase in which the conversion rules are different between the recordingsystem and the reproducing system is as follows. In the recordingsystem, a Fourier transform recording is carried out by a Fouriertransform lens optical system. However, in the reproducing system, thefollowing cases are included: the case in which conversion is performedusing not only the inverse Fourier transform lens optical system butalso an additional optical system to obtain an intermediate data anddemodulation is performed; and the case in which a detected intermediatedata is inverse Fourier-transformed by a computer and a predetermineddata is thereby demodulated instead of using the inverse Fouriertransform lens.

In the hologram recording and reproducing system of this embodiment, aconversion table is defined in advance. Examples of the conversion tableare an inverse Fourier computing device, one in which a Fouriertransform pattern in the vicinity of the Fourier plane is uniquelyassociated with a data that has not yet been Fourier-transformed, one inwhich a positional data that is output from a predetermined positionsensor is uniquely associated with each of the data that are recorded ina reference data-holding hologram, etc. Various conversion tables aredefined in advance for other recording medium formats, and theconversion tables are recorded in a non-volatile memory of the recordingand reproducing system upon shipment. It is also possible to record theconversion tables in a rewritable memory.

FIG. 3 shows an example of a first embodiment of a recording andreproducing system according to the invention.

In this embodiment, the inverse Fourier transform lens is not used, asshown in FIG. 3; a light-receiving face of a photo-detector 200 such asa two-dimensional light sensor or the like is disposed in the vicinityof the Fourier plane FF, and a recording medium 10 is disposed in theupstream of the photo-detector 200, that is, between the photo-detector200 and a Fourier transform lens 16. In addition, the recording andreproducing system has a similar configuration to that of theconventional one except that the system is equipped with the inverseFourier computing device and a non-volatile memory ROM that is connectedto a controller 30 and stores a conversion table in which a Fouriertransform pattern in the vicinity of the Fourier plane is associatedwith a data that has not yet been Fourier transformed. At the time ofreproducing, the controller 30 computes a predetermined original datafrom a reproduced Fourier transform pattern according to the inverseFourier computing device. It should be noted that the photo-detector 200is sufficient as long as it can obtain the Fourier transform pattern asintermediate data, and the position of the photo-detector 200 may be inthe vicinity of either the front or the back of the Fourier plane.

First, at the time of recording, a light beam emanating from a laserlight source 11 is split by a beam splitter 13 into two beams, a signallight beam that propagates linearly and a reference light beam thatdeflects upward. The respective beams are guided to respective lightpaths of signal and reference light beam optical systems.

The signal light beam 12 a that has passed through the beam splitter 13goes through a shutter 6 a, a light beam expander BX, a spatial lightmodulator SLM, and a Fourier transform lens 16, and enters a recordingmedium 10. The time during which the signal light beam 12 a is projectedto the recording medium is controlled by the automatic shutter 6 a,which is controlled by the controller 30, and the signal light beam isenlarged by the beam expander BX into a collimated light having apredetermined diameter. The spatial light modulator SLM is, for example,a transmissive LCD with a two-dimensional plane of 480 pixelsvertically×640 pixels horizontally, and converts the light beam from thebeam expander BX into signal light according to a digital recording datasupplied from an encoder 25. For example, when the data displayed on thespatial light modulator SLM is the image data A shown in FIG. 2( a) andlight transmits through that portion, turning to signal light, the imagedata A is Fourier-transformed and a Fourier transform pattern as shownin FIG. 4 is generated in the vicinity of the Fourier plane FF.Accordingly, the data is recorded in the recording medium 10 as aninterference pattern of the reference light and the signal light thathas not yet reached the Fourier transform pattern. Generally, by thespatial light modulator SLM, a data is spatial-modulated according to arecording page data into a two-dimensional dot pattern in which eachpixel is transmissive or non-transmissive; thereafter, it isFourier-transformed by the Fourier transform lens 16, gathered into therecording medium 10, and formed into a point image having a high lightintensity on the Fourier plane FF. Therefore, it is preferable that therecording medium 10 is disposed in the vicinity of the Fourier plane FF.

The recording medium 10 has, for example, a disk-like shape or athin-plate-like shape, comprising a photorefractive polymer. In the caseof the disk recording medium, the recording medium 10 is placed on arotation table (not shown in the drawings), and the rotation table isdriven by a drive unit that drives the rotation table around therotational symmetry axis as its center. The drive unit is so configuredthat the rotation of the table or the like is controlled by thecontroller 30. According to a signal corresponding to aposition-determining data from a photo-detector, the controller 30controls the rotation position by driving the rotation table with astepper motor or the like, and controls the relative position of therecording medium 10 with the signal-generating unit and the detectorunit by shifting either the recording medium 10 or the signal-generatingunit and the detector unit with a mechanism not shown in the drawings.

On the other hand, in the reference light beam optical system, areference light beam 12 b is reflected by mirrors 18 and 19 andprojected to the recording medium 10. The reference light beam 12 b isbrought to intersect and cause interference with a signal light beam 12a from the lens 16 in a position inside the medium so that athree-dimensional interference pattern is formed. Thus, when recordingdata, the signal light and the reference light are simultaneouslyprojected to a predetermined location in the recording medium 10, andthe interference pattern is recorded as a refractive index grating inwhich the refractive index has been changed, as in the conventionalsystem. The formation time of a hologram is controlled by releasing ofthe automatic shutter 6 a.

Thus, information that is in the middle of Fourier transform is recordedin the recording medium 10. At the time of reproducing in theembodiment, inverse Fourier transform by means of an optical system isnot carried out. A reproduced data from a hologram is reproduced as aFourier transform pattern on the two-dimensional photo-detector 200 whena two-dimensional photo-detector 200 is disposed in the vicinity of theFourier plane, and therefore, an output from the two-dimensionalphoto-detector 200 is computed based on a conversion table in anon-volatile memory ROM by the controller 30 according to inverseFourier transform; and thus, an original data is obtained. In thisconfiguration, an optical system of inverse Fourier transform lens isnot required, and the size of the recording and reproducing system canbe reduced. Such a conversion table can also include algorithms for dataconversion and the like.

FIG. 5 shows one example of a second embodiment of the recording andreproducing system according to the invention.

In this embodiment, as shown in FIG. 5, an inverse Fourier transformlens 16 a is used unlike the first embodiment, and a referencedata-holding hologram 299, not a photo-detector, is disposed at thefocal point position. The reference data-holding hologram 299 generatesdiffracted light that corresponds to the reference light beam in which areference data hologram is recorded, on a position sensor 300 disposedat a position spaced apart by a predetermined distance. This recordingand reproducing system has a similar configuration to that of theconventional 4f system hologram recording system except the following;it is equipped with the reference data-holding hologram 299 and theposition sensor 300, and it is also equipped with a non-volatile memoryROM that is connected to the controller 30 and stores data of aconversion table in which a positional data (x y data) that is outputfrom the position sensor 300 corresponding to a spot of the referencelight beam on the position sensor 300 and each data recorded in thereference data-holding hologram are uniquely associated, as shown inFIG. 6. Then, at the time of reproducing, the controller 30 computes apredetermined original data from the reproduced positional dataaccording to the conversion table.

An operation of the 4f system hologram recording system of the secondembodiment is described.

First, all the dot patterns that are produced by the spatial lightmodulator SLM or the portions for several pages to be used in recordingare subjected to angle multiplexing, and a reference data hologram isformed in advance in the reference data-holding hologram 299 as apre-format by a device not shown in the drawings. Then, as shown in FIG.5, the reference data hologram 299 is disposed at the focal pointposition of the inverse Fourier transform lens 16 a. In addition, aconversion table in which the respective angle values of the referencelight in the angle multiplexing during the formation of the referencedata-holding hologram 299 and all the dot patterns are associated isrecorded in the non-volatile memory ROM of the recording and reproducingsystem in advance.

Next, at the time of recording, a refractive index grating correspondingto the dot pattern of the spatial light modulator SLM is recorded intothe recording medium 10 using the signal light and the reference lightas usual.

Next, at the time of reproducing, when the recording medium 10 isreproduced using a predetermined reference light, signal light isreproduced as usual and the signal light is projected into the referencedata-holding hologram 299. Then, a diffracted light corresponding to thereference light having the angle recorded during the pre-formatting isgenerated as an intermediate data from the reference data-holdinghologram 299, and it is detected by the position sensor and comparedwith the conversion table that is stored in the non-volatile memory ROMof the recording and reproducing system in advance; and thus, a desireddot pattern data is reconstructed.

Accordingly, even without using an expensive two-dimensional detectorsuch as CCD as used in conventional cases, the configuration is possiblewith the position sensor 300, which is inexpensive. Furthermore, the CCDperforms the transfer of electric charges (data) pixel by pixel andtherefore cannot perform high-speed information transfer, but theposition sensor 300 of this embodiment can perform high-speed detectionand transfer of information.

It should be noted that the recording and reproducing system can also beconfigured by using recording media having a shape of body ofrevolution, such as circular cylinder, and recording media such as cardsor the like, although a disk recording medium 10 is used in theforegoing example.

1. A hologram recording and reproducing system comprising: a supportingunit for freely attachably supporting a recording medium including aphotosensitive material; a signal light-generating unit for projecting acoherent light beam modulated according to a predetermined data into therecording medium and generating a diffraction grating by providing athree-dimensional light interference pattern in the recording medium; adetector unit for detecting and photoelectrically converting adiffracted light from the diffraction grating; and a demodulating unitfor demodulating a predetermined data from an output from the detectorunit, the hologram recording and reproducing system characterized inthat: the detector unit has an intermediate data-generating unit forgenerating an intermediate data, and the demodulating unit has aconversion table in which the intermediate data and the predetermineddata are uniquely associated, and demodulates the predetermined data byperforming an operation based on a correlation in the conversion table,and in that: at the time of recording of the hologram recording andreproducing system, the predetermined data of image is recorded in therecording medium as the interference pattern of Fourier transformedpattern formed with an optical system, and at the time of reproducing,inverse Fourier transform is performed by the detector unit and thedemodulating unit to reproduce the predetermined data of image.
 2. Thehologram recording and reproducing system according to claim 1,characterized in that the signal light-generating unit includes areference light-generating unit for projecting a coherent referencelight beam being the coherent light beam and having a first wavelengthinto the recording medium, modulates a coherent signal light beam beingthe coherent light beam and having the first wavelength according to thepredetermined data, illuminates the recording medium with the signallight beam so that the signal light beam intersects the reference lightbeam within the recording medium, and generates a three-dimensionallight interference pattern with the reference light beam.
 3. Thehologram recording and reproducing system according to claim 1 or 2,characterized in that the signal light-generating unit has a spatiallight modulator, the detector unit has a photo-detector being theintermediate data-generating unit for generating the intermediate data,a light-receiving face of the photo-detector being disposed in thevicinity of a Fourier plane, and the recording medium is disposed in anupstream of the photo-detector.
 4. The hologram recording andreproducing system according to claim 1 or 2, characterized in that thesignal light-generating unit has a spatial light modulator, and thedetector unit has an inverse Fourier transform lens, a referencedata-holding hologram disposed at a focal point position of the inverseFourier transform lens, and a position sensor that receives a diffractedlight from the reference data-holding hologram and is disposed at aposition spaced apart by a predetermined distance from the referencedata-holding hologram, the position sensor being the intermediatedata-generating unit for generating the intermediate data.